ARM: dts: imx51 cpu node

This patch adds a cpus/cpu@0 node for imx51 with default operating
points for the cpufreq-cpu0 driver. There is currently no regulator
support, so the voltages are 0 here.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>